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CROP INSIGHTS Sustainable Corn Stover Harvest for Biofuel Production by Mark Jeschke and Andy Heggenstaller, Agronomy Research Managers

Summary • Corn stover is the most plentiful source of lignocellulosic biomass in the U.S. Sustainable utilization of corn stover as a feedstock for ethanol and other biofuels could help meet energy needs while delivering agronomic benefits. • In fields where excess residue interferes with planting, impedes stand establishment, and ties up nitrogen, partial stover harvest can increase corn yields and potentially reduce production costs. • Sustainable corn stover harvest requires that only a portion is removed from the field, leaving a sufficient amount behind to meet other critical needs, including mitigation of soil erosion, maintenance of soil organic matter, and sustained soil fertility. Higher corn yields have been accompanied by increases in corn residue. Harvesting excess stover for ethanol pro- • The amount of stover that can be sustainably harvested is duction is a way to capture additional value from the crop generally most limited by the amount that must be left in while reducing residue management challenges. the field to maintain soil organic matter levels. Sustainable corn stover harvest must be done in such a way • Crop nutrients, most notably potassium, are removed as to provide value to the grower and biofuel producer from the field when stover is harvested. Specific removal without negatively impacting the health and productivity of rates will vary according to soil nutrient levels, growing the soil. Stover removal is not an option for every field, conditions, hybrid, and the time and method of harvest. particularly those subject to erosion or those with low average yields and low stover production. On the other Introduction hand, stover harvest may offer stand establishment and yield advantages for uniform, high yielding fields with high levels Lignocellulose is a structural component of all plant matter. of corn residue that impede crop production. Weighing the Because it is abundant in nature, has high energy value, and crop production advantages versus the value of the residue is not used directly for human consumption, this material removed can help growers make a profitable and sustainable holds great promise for renewable energy production. The decision for each field. primary plant materials being considered for large-scale cellulosic ethanol production include crop residues, “energy Value of corn stover in a cropping system crops” developed specifically for fuel, and forest and wood processing residues. Of these, crop residues, and particularly Corn residue plays several important roles in a cropping those of corn, hold the greatest potential (U.S. Department system. Its most obvious function, and the primary driver of Energy, 2011). behind the historical trend toward reduced and no-tillage, is Corn produces the highest volume of residue of all the mitigating wind and water erosion. Return of corn residue to major crops in the U.S., and this volume has increased in the soil is also important in maintaining soil organic matter, tandem with corn grain increases. Because of its abundance which in turn influences soil structure, fertility, and water and even excess in some areas, several “second-generation” holding capacity. Because of the value that corn residue ethanol plants that use corn residue as a feedstock are provides in maintaining soil productivity and maximizing planned for production in the U.S. Corn Belt. This will grain yields, a sustainable corn stover harvest program provide growers with the opportunity to get more value necessarily only removes a portion of the total corn stover, from their agricultural land by supplying corn stover to leaving a sufficient amount behind to meet critical needs make biofuel. (erosion control, fertility, soil carbon, etc.).

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Corn residue higher, often excessive production is not substantially different than tilling the soil to increase decomposition (Al-Kaisi and Yin 2005). The level of corn residue remaining in the spring has In cases where management of excess residue is a driving increased significantly in many fields as a result of the factor in management decisions, partial stover harvest could general trend for higher corn yields (Figure 1). Management potentially expand rotation and management options. For of corn residues has become more difficult due to the use of example, a reduction in excess residue could allow for foliar fungicides and Bt traits that result in stronger corn increased production of corn following corn, particularly in stalks that resist decomposition. Adoption of reduced tillage the northern Corn Belt where residue decomposition tends practices has also contributed to less residue breakdown in to be slower. Tillage or other field operations currently done many areas. Corn produced in the U.S. Corn Belt has specifically to manage residue could potentially be maintained a relatively stable harvest index of 0.5, meaning eliminated, providing substantial production cost savings. that increases in grain yield have been accompanied by Partial stover harvest could have secondary benefits as well, corresponding increases in stover production (Lorenz et al. including reduction in inoculum levels for corn pathogens 2010). that overwinter in corn residue (e.g., anthracnose, gray leaf 450 10.7 spot, Goss’s wilt, and northern leaf blight) as well as Yield Record: 429 bu/acre reduced nitrogen immobilization. 400 9.5 ) (tons/acre Yield Stover

350 NCGA National 8.3 Residue Management Yield Contest 300 Winners 2006-2011 7.1 Research conducted by the

( bu /acre ) 250 5.9 University of Missouri and Pioneer Hi-Bred compared 200 4.7 several common methods 150 3.6 of managing residue in 100 2.4 no-till continuous corn Grain Yield Yield Grain History 50 1.2 including (1) use of row FAPRI Baseline cleaners at planting, 0.0 0 (2) fall nitrogen application, (3) fall stalk chopping and (4) baling and removing 50% 1966 1970 1974 1994 1998 2002 2006 2010 2014 2018 1978 1982 1986 1990 of stover following grain harvest. Over the four year Figure 1. Average U.S. corn grain and stover yields and duration of the experiment, only the 50% stover removal recent NCGA contest-winning yields. Estimates of corn treatment provided a significant yield advantage compared stover yield are based on a harvest index of 0.5. to no residue management (Figure 2). This outcome was partially explained by improved stand establishment In many highly productive systems, particularly under following stover removal (data not shown), but was also continuous corn, corn stover production most often exceeds likely due to reduced nitrogen immobilization and enhanced the minimum amounts needed to maintain soil health and early corn growth. productivity, making sustainable stover harvest a viable option. In systems where excessive residue levels have 180 become a management challenge, removing a portion of the 174 residue should have a positive impact on stand 170 establishment, early growth and grain yield. 160 157 Advantages of removing excess residue 150 154 151 152

The primary advantage in reducing surface residue is 140 preventing its interference with planting and stand Grain Yield (bu/acre) Yield Grain establishment of the subsequent crop. In many high-yielding 130 areas of the U.S. Corn Belt, residue accumulation has become an increasing problem. To counter this problem, 120 many growers are chopping stalks during or after harvest None Row Fall N Fall Stalk Bale and and/or incorporating stalks into the soil through more Cleaners Chop Remove aggressive tillage. Both practices increase microbial Residue Management degradation of stalks with resulting loss of carbon through CO2 release. From a carbon sequestration standpoint, Figure 2. Effect of residue management practices on grain managing excess residue by removing a portion for ethanol yield in no-till continuous corn (Wiebold, 2011).

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Nitrogen Availability Soil Moisture and Temperature One of the major impacts of excess corn residue is reduced Reducing the amount of nitrogen availability as a result of microbial uptake and residue on the soil surface is immobilization. In order to break-down crop residues, soil generally associated with in- microbes require a nitrogen source. While this immobilized creased soil temperature in the nitrogen is later released back to the soil as microbial spring and a faster rate of soil populations decline, it is unavailable during spring and early drying, which can be advan- summer when crop nitrogen demands are highest. For tageous in many cases. Higher residues that have a carbon-nitrogen (C:N) ratio less than soil temperatures in the spring 30, ample nitrogen for microbial decomposition is derived will almost always be positive, directly from the residue itself and no immobilization since soil temperatures at occurs. This is the case for soybean residues as well as for planting are often below the most animal manures. For residues and other organic optimum levels for germination amendments that have a C:N ratio greater than 30, and emergence. Warmer soil additional nitrogen required for decomposition is derived temperatures can improve from soil mineral nitrogen that would otherwise be available germination as well as subsequent root growth and nutrient for crop uptake. Corn residues typically have a C:N ratio in uptake. the range of 50-75 and can immobilize large quantities of Faster soil drying is usually positive, especially in northern nitrogen, especially at high residue levels. states and northern areas of central states where wet spring Research conducted at the University of Illinois conditions often prevail. Faster drying soils allows growers demonstrated the value of corn stover harvest as means of to get into the field sooner for spring tillage, fertilizer reducing nitrogen immobilization and fertilizer nitrogen application and planting. However, in moisture-limited requirements in continuous corn production (Figure 3). environments such as the semiarid Plains states, reduced Researchers determined optimum nitrogen fertilizer rates for soil moisture associated with residue removal can have a continuous corn under chisel plow and no-till in a five-year negative impact on yield. Research conducted in Nebraska study that included multiple field locations. Compared to no found significant yield reductions with residue removal, stover harvest, optimum nitrogen rate for the no-till system which was attributed primarily to reduced soil moisture was reduced by 47 lb/acre when half of the stover was (Wilhelm et al. 1986). In these chronically drought-stressed removed and 65 lb/acre when all of the stover was removed. environments, corn stover harvest may not be feasible. The For chisel plow, optimum nitrogen rate was reduced by 9 net effect of partial stover harvest on grain yield due to its and 16 lb/acre, respectively, with half and complete stover influence on soil moisture will vary according to conditions, removal. Under no-till, corn yield was also increased by 5 and is likely to have neutral or positive effects in most cases bu/acre when half of the stover was removed and 18 bu/acre where available moisture is adequate. when all of the stover was removed. Yields under chisel plow were similar across stover harvest treatments. How much residue can be harvested?

250 Excessive levels of corn residue in many fields make Chisel Plow No Till residue harvest a viable, sustainable option. In these fields, 240 the important question is how much of the residue can be 225 removed. That question can be best answered by examining the factors that control how much residue should remain. Those factors include soil erosion prevention, soil organic 200 carbon maintenance, and soil fertility management. 195 193 186 Soil Erosion Mitigation 175 179 175 Soil erosion is a major consideration in determining the

Optimum N N (lb/acre) Rate Optimum potential for sustainable corn stover removal. Any stover 150 harvest program must leave enough residue on the soil to None Half All mitigate water and wind erosion, keeping soil loss within Stover Harvest tolerable levels. Tools such as RUSLE2 (Revised Universal Soil Loss Equation, version 2) and WEPS (Wind Erosion Figure 3. Effect of stover harvest and tillage system on Prediction System) are available to assist in developing a fertilizer nitrogen requirements for continuous corn soil conservation plan. (Nafziger 2011).

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Soil Organic Carbon Maintenance Another major consideration in determining the potential for sustainable corn stover harvest is the need to maintain soil organic matter levels. Soil organic matter is important to soil structure, water-holding capacity and nutrient supply to plants. Soil organic matter levels are determined by the rate of loss through erosion and mineralization and the rate of gain through return of plant residue and other organic material. Corn stover harvest can potentially influence both the input and output of organic matter from the soil so it is important to determine how much can be safely removed without having a negative impact. Soil organic matter plays many critical roles in producing high grain yields, creating an economic incentive for ensuring that corn stover harvest is done in a sustainable manner.

The amount of corn residue needed to manage erosion can Soil organic matter is frequently measured according to its vary greatly according to field characteristics and manage- carbon fraction, or soil organic carbon. Research indicates ment practices (Table 1), but is often considerably less than that maintenance of soil organic carbon is generally likely to the total quantity produced (Figure 1). Factors such as soil be the most limiting factor on the amount of corn stover that type, slope, crop rotation, and tillage all influence the can be sustainably removed; i.e., the amount of residue amount of residue needed to mitigate water and wind needed to maintain soil organic carbon will likely be greater erosion. Consequently, the potential for sustainable stover than the amount needed to mitigate erosion (Figure 4). harvest will also vary based on field characteristics, and will not be feasible in all situations. In some cases, changes to crop management practices (such as continuous corn, no-till, or rotational stover removal) may make it possible to accommodate partial stover harvest in a cropping system.

Table 1. Examples of residue levels needed to keep water erosion within tolerable limits as determined by RUSLE2 for several Midwestern sites using conservation tillage1.

Corn- Corn- State Soil Slope Corn Soybean % --- tons/acre --- Kossuth silty clay IA 0-2 0.41 0.61 loam Figure 4. Average amounts of corn residue needed to maintain soil organic carbon (Johnson et al. 2006) and IL Fox silt loam 2-4 0.60 0.80 manage water and wind erosion (Wilhelm et al. 2007) Morley-Glywood IN 1-4 0.55 0.73 across multiple sites. Complex loam MI Kalamazoo loam 0-6 0.37 0.37 Residue levels needed to maintain soil organic carbon will vary according to soil characteristics and management Port Bryon silt MN 2-6 0.03 0.04 factors; therefore sustainable removal rates will need to be loam determined on a site-specific basis. Yield level and crop MN Hayden loam 2-6 0.04 0.04 rotation both greatly influence the amount of residue needed to maintain soil organic carbon levels (Table 2); as does NE Holdrege silt loam 3-5 0.04 0.43 tillage (Figure 3). The Soil Conditioning Index (SCI) is a OH Mermill loam 0-2 0.48 0.63 tool used to predict qualitative changes in soil organic Moody-Nora silty matter based on management practices and has proven to be SD 2-6 0.03 0.03 clay loam effective for estimating sustainable corn stover removal rates. Continued increases in corn grain yield will be WI Dresden silt loam 2-6 0.36 0.60 accompanied by increases in stover production, which will 1 Wilhelm et al. 2007 expand opportunities for sustainable corn stover harvest.

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Table 2. Effects of yield level and crop rotation on quantity Table 3. Estimates of the nutrient content of corn stover. of corn stover available for continual harvest while maintaining soil organic carbon. (Estimates based on data from Source N P2O5 K2O multiple research sites; actual removal rates will vary). --- lbs/ton dry matter --- 1 Stover Available for Harvest Iowa State 15.0 5.9 25.0 Michigan State2 13.6 3.6 19.7 Grain Stover Continuous Corn- 3 Yield Production1 Corn2 Soybean3 Univ. of Wisconsin 12.1 3.3 35.7 4 bu/acre ------dry matter tons/acre ------Univ. of Illinois - 7.0 30.0 5 150 3.5 1.2 0.0 DuPont – rake and bale - 4.4 23.5 Iowa State – single-pass 160 3.8 1.5 0.3 9.5 3.0 16.5 bale6 170 4.0 1.7 0.5 1 Sawyer et al. 2011 180 4.3 2.0 0.8 2 Gould 2007 3 190 4.5 2.2 1.0 Bundy 1998 (based on 4.2 tons dry stover/acre at 150 bu/acre) 4 Fernández 2007 200 4.7 2.4 1.2 5 Iowa corn stover test harvest 2010 6 Combine with integrated baler 2011 210 5.0 2.7 1.5 220 5.2 2.9 1.7 230 5.4 3.1 1.9 240 5.7 3.4 2.2 250 5.9 3.6 2.4 1 Based on a harvest index of 0.5 2 Estimated 2.3 tons/acre dry corn stover needed to maintain soil organic carbon under continuous corn with conservation or no-tillage (Johnson et al. 2006, Wilhelm et al. 2007). 3 Estimated 3.5 tons/acre dry corn stover needed to maintain soil organic carbon under corn soybean rotation with conservation or no- tillage (Johnson et al. 2006, Wilhelm et al. 2007).

Soil Fertility Management

Stover harvest increases the total amount of plant material removed from a field, resulting in greater quantities of 35 nutrients also being removed. Estimates of stover nutrient content are available in various state soil fertility extension publications (Table 3). Though these estimates are useful as 30 a general guide, the actual impact of stover removal on soil fertility is a complex issue that can be affected by soil nutrient levels, growing conditions and hybrid, as well as 25 the time and method of stover harvest. Nutrient removal estimates based on silage harvest or for 0 Content 0 Content (lbs/ton) 20 corn stover at physiological maturity typically overestimate 2 the amount of nutrients actually removed by corn stover harvest. Generally, research has shown that the nutrient 15 content of corn stover decreases between physiological K Stover maturity and grain harvest (Sawyer and Mallarino 2007, Johnson et al. 2010). Nutrient removal decreases further if 10 stover harvest occurs days or weeks following grain harvest, 25-Sep 5-Oct 15-Oct 25-Oct 4-Nov particularly if the stover is rained on prior to collection. Figure 5. Decline in corn stover K O content observed Rainfall can leach nutrients out of corn stover, particularly 2 during 2010 DuPont stover test harvest in Iowa. Each point potassium, which exists in a soluble form in the plant represents average K O content of five large, square bales. (Figure 5). 2

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Partial stover harvest will increase the removal of plant Coulter, J.A., and E.D. Nafziger. 2008. Continuous Corn Response nutrients; however, short and long term effects on soil to Residue Management and Nitrogen Fertilization. Agron. J. fertility will vary for different nutrients. In general, growers 100:1774-1780. should account for phosphorus and potassium removal in Fernández, F. 2007. What is the Nutrient Value of Corn Stover their fertility programs, keeping in mind that nutrient Removal? The Bulletin 23-9. University of Illinois content values from university extension recommendations http://bulletin.ipm.illinois.edu/article.php?id=860 designed for silage harvest usually overestimate the amount actually removed - especially for potassium - depending on Gould, K. 2007. Corn Stover Harvesting. Cattle Call 12-2. when and how stover is harvested. Long-term effects on Michigan State University. http://www.beef.msu.edu/LinkClick.aspx?fileticket=q0jUXM2 nitrogen, sulfur, and other nutrients are not as readily MIqM%3d&tabid=537 apparent; but fertility should be monitored if partial stover harvest becomes a regular, long-term cropping practice. Johnson, J.M.-F., R.R. Allmaras, and D.C. Reicosky. 2006. Estimating Source Carbon from Crop Residues, Roots and Stover removal has compensating effects on nitrogen Rhizodeposits Using the National Grain-Yield Database. Agron. fertilizer needs. Stover harvest removes a small amount of J. 98:622-636. nitrogen from the field, but also reduces the amount of nitrogen immobilized in the soil the following year. In corn- Johnson, J.M.F., W.W. Wilhelm, D.L. Karlen, D.W. Archer, B. after-corn fields, growers must increase their nitrogen Wienhold, D.T. Lightle, D. Laird, J. Baker, T.E. Ochsner, J.M. fertilizer rates (vs. corn after soybeans) to compensate for Novak, A.D. Halvorson, F. Arriaga, N. Barbour. 2010. Nutrient Removal as a Function of Corn Stover Cutting Height and Cob nitrogen immobilized for residue decomposition. Removing Harvest. Bioenerg. Res. 3:342-352. a portion of the corn stover can reduce the amount of additional nitrogen fertilizer required because less nitrogen Lorenz, A. J., T.J. Gustafson, J.G. Coors, N. de Leon. 2010. is tied up in the spring by residue decomposition (Coulter Breeding Maize for the bioeconomy: A literature survey and Nafziger 2008). It is unclear whether partial stover examining harvest index and stover yield and their relationship removal will reduce the amount of organic nitrogen to grain yield. Crop Science. 50: 1-12 available through mineralization over the long-term. It Nafziger, E.D. 2011. Tillage and Nitrogen Responses to Residue stands to reason that soil nitrogen supply capacity will Removal in Continuous Corn. Proceedings of the North Central depend upon maintaining organic matter levels through Extension-Industry Soil Fertility Conference. Vol. 27. IPNI. sustainable, partial stover harvest. Sawyer, J.E. and A.P. Mallarino. 2007. Nutrient removal when harvesting corn stover. IC-498(22) Iowa State University Extension. http://www.ipm.iastate.edu/ipm/icm/2007/8- 6/nutrients.html

Sawyer, J.E., A.P. Mallarino, R. Killorn, S. K. Barnhart. 2011. A General Guide for Crop Nutrient and Limestone Recommendations in Iowa. PM1688. Iowa State University Extension. http://www.extension.iastate.edu/Publications/PM1688.pdf

U.S. Department of Energy. 2011. U.S. Billion-Ton Update: Biomass Supply for a Bioenergy and Bioproducts Industry. R.D. Perlack and B.J. Stokes (Leads), ORNL/TM-2011/224. Oak Ridge National Laboratory, Oak Ridge, TN. 227p.

Wiebold, W.J. 2011. Mitigation of Stover Effects on Yield in Continuous Corn Planted without Tillage. Pioneer Crop Sources Management Research Awards Update.

Al-Kaisi, M.M., and X. Yin. 2005. Tillage and Crop Residue Wilhelm, W.W., J.W. Doran, and J.F. Power. 1986. Corn and Effects of Soil Carbon and Carbon Dioxide Emission in Corn- soybean yield response to crop residue management under no- Soybean Rotations. J. Environ. Qual. 34:437-445. tillage production systems. Agron. J. 78:184-189.

Bundy, L. G. 1998. Corn Fertilization. University of Wisconsin- Wilhelm, W.W., J.M.F. Johnson, D.L. Karlen, and D.T. Lightle. Extension A3340. 2007. Corn Stover to Sustain Soil Organic Carbon Further http://www.soils.wisc.edu/extension/pubs/A3340.pdf Constrains Biomass Supply. Agron. J. 99:1665-1667.

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